1. Field of the Invention
This invention relates to frequency translation of voice-frequency signals, and, in particular, to frequency translation in which the carrier leak is kept to a minimum without the use of band-stop filters.
2. Background Description
In communication circuits, and in particular, telephone communication circuits which employ frequency division multiplexing, it has always been important to use a minimum of bandwidth for each communication channel. This has most often been accomplished by use of the single-sideband modulation techniques in which a balanced modulator is employed, and the desired output sideband selected by a filter. Considerable effort has been made in the design of modulation circuits so as to reduce carrier-leak to a minimum, i.e., the carrier frequency which leaks through the modulator and is present along with the sidebands generated by the modulation process. While this problem is of considerable interest where standard 4 KHz channel spacing is employed, this interest stems primarily from a loading standpoint, because the passband frequencies for the channel are positioned such that the carrier leak does not adversely affect the communication channel. This is not the case in submarine cable systems which employ 3 KHz spacing because the carrier leak in the one channel here will cause an audible tone in another channel. For this reason the carrier leak must be maintained at a very low level (typically 1/30,000 of the signal level). Techniques for the design of channel equipment to operate in this restricted channel bandwidth are discussed in the following articles: "Sixteen-Channel Banks for Submarine Cables", R. S. Tucker, Bell Laboratories Record, July, 1960, pp. 248-252; and "Channel Equipment Design for Economy of Band-Width", H. B. Law, et al., The Post Office Electrical Engineers Journal, 53, Part II, (July, 1960), pp. 112-117. These articles disclose information on compatible systems which were designed for submarine cable applications in which a channel bandwidth of 3 KHz is employed. A band-stop filter was required to reduce the carrier leak to an acceptable level. A number of disadvantages are apparent from the use of the band-stop filter. Such things as cost and complexity, which add to the cost of the equipment and the adverse effect of the filter characteristic on the passband, which latter effect must be corrected by frequency attenuation equalizers, thus adding additional cost and complexity.